In recent years there has been a rapid progress in portable and cordless designs of electronic equipment such as audio-visual equipment and personal computers. As the power source of these equipment, non-aqueous electrolyte (organic solvent type liquid electrolyte) secondary batteries as represented by various types of high-capacity alkaline storage batteries and lithium-ion secondary batteries are suitable. Furthermore, as a result of an effort of developing sealed type versions with a high energy density and superior load characteristic, sealed type non-aqueous electrolyte secondary batteries are in wide use as the power source of portable equipment including watches and cameras.
Now, with the non-aqueous electrolyte secondary battery, a chemical change in the power generating elements inside the battery takes place in the event of failure of applied equipment including the charger, overcharge, or misuse. For instance, an abnormal reaction due to overcharge or short circuit decomposes the electrolyte or active materials, thus causing unusual gas evolution inside the battery and an excessive internal pressure of the battery. For this reason this type of batteries has to date been provided with the following explosion-proof mechanism. That is, in the event the battery internal pressure increases beyond a designed value, a vent member that has been exerted with the internal pressure is pressed toward the direction of the internal pressure (the direction of diffusion of the internal pressure) and deformed, thereby causing rupture of a thin portion of electrically conducting member or separation of the weld between the vent member and the electrically conducting member thus cutting off the electric current in the initial stage of occurrence of overcharge or short circuit and stopping the abnormal reaction. As a result, an increase in the battery temperature or battery internal pressure due to charging current or short-circuit current can be controlled and safety of the battery is secured.
The battery structure disclosed in U.S. Pat. No. 4,943,497 is valuable in that it provides a commercially useful product which is fully protected internally against overcharge. The gas evolution mechanism which activates the cutoff device depends on the voltage and temperature of the battery and on time. The speed of gas evolution is not constant; it increases with increasing voltage and temperature of the battery. It is of special significance to note in actual use of the battery that evolution of a gas continues with time at predetermined voltage and temperature. While it is required that evolution of a gas during overcharge assures safe stoppage of battery operation, sustained evolution of the gas in normal operation must be avoided. Unless gas evolution can be avoided, decomposition product of the gas will accumulate with time causing an increase in the gas pressure presenting a possibility of operation of the cutoff device in normal use.
By a proper selection of the liquid electrolyte and positive active material, safety requirements can actually be satisfied to a certain degree. However, these requirements hinder the selection because of such other reasons as cost, complexity, and energy density. Furthermore, when the gas evolution continues in a completely sealed battery, the maximum life of the cutoff device will be limited.
The prime object of the present invention is to provide a safe and explosion-proof non-aqueous electrolyte secondary battery which is equipped with a cutoff device for cutting off electrical connection within the battery with safety and certainty in the event of overcharge while remaining inoperable in normal use or storage at an elevated temperature, and which is free from the danger of explosion by combination and optimization of safety devices and superior in safety in the event of evolution of a gas in excess of the gas discharge capacity as a result of abandonment or throwing into fire of the battery.